Optical recording disks have recording layers on substrates. Recording and reproducing operation is generally carried out by directing a laser beam or similar light to the recording layers through the substrates. Therefore, the substrates used are formed of transparent materials, for example, glass and resins. For a weight reduction and ease of formation of tracking grooves and pits, conventional optical recording disks used resinous substrates. Polymethyl methacrylate and polycarbonate were frequently employed.
Polymethyl methacrylate has excellent optical properties including a low coefficient of optical elasticity and a very low double refraction or birefringence, but is poor in dimensional stability due to high water absorption and low heat resistance.
Therefore, polycarbonates having low water absorption and relatively high heat resistance are used most often. However, the polycarbonates have a high coefficient of optical elasticity and often high birefringence since their polymer chain tends to orient in a flow direction. For achieving a higher recording density and higher transfer rate, it was attempted in recent years to use disks of a larger diameter of at least 200 mm and carry out recording/reproducing operation at a high number of revolutions of at least 1,800 rpm. Injection molding of polycarbonates into such large diameter substrates incurs birefringence which is further increased through application of stresses during high speed rotation, resulting in a lowering of reproduced outputs. The polycarbonates are not fully low in water absorption so that large size substrates will undergo substantial distortion. Some lack of rigidity causes axial runouts and focus tracking errors during high rotational speed driving at 1,800 rpm or higher, especially at 3,000 rpm or higher. In addition, the recording layers can be corroded by potential penetration of moisture through polycarbonate substrates.
To meet the recent demand for high rotational speed driving at 1,800 rpm or higher, especially at 3,000 rpm or higher, substrates having higher mechanical strength should be used. Glass substrates have the high risk of failure when the disks are accidentally dropped or hit. Particularly when chemically tempered glass of high strength is used, it can fracture into scattering fragments. Glass substrates also have the problem that grooves useful as a pre-format information carrier cannot be formed.
To protect the recording layer from mechanical damage as by scratching, it is convenient to provide a protective plate on the recording layer or to mate a pair of substrates such that the recording layer is inside.